Hybrid Electric Vehicles (HEVs) may use various types of powertrain architectures to provide parallel, series, or a combination to transfer torque from two or more sources to the traction wheels. A power split architecture combines the driving torque generated by the engine and the torque generated by one or more electric machines in various operating modes. A representative power split architecture is illustrated in FIG. 1. The two electric machines, referred to as the motor and the generator, may be implemented by permanent-magnet AC motors with three-phase current input. The engine and the generator may be connected by a planetary gear set with the engine crankshaft connected to the carrier and the generator rotor connected to the sun gear. The gear on the motor output shaft may be meshed to the counter shaft with a fixed ratio. The ring gear may also be connected to the counter shaft in a fixed ratio arrangement.
In this example, the motor is connected to the driveline through the countershaft in parallel to the engine-sourced torque output from the ring gear. The main functions of the motor include: 1. Drive the vehicle in electric drive mode by supplying full required torque; 2. Compensate the ring gear torque output based on driver commands; and 3. Damp driveline oscillation.
Vehicle drivability, including smooth vehicle operation, is a challenging issue for all types of automotive implementations. Driveline resonance is one of the major reasons that a driver feels unsmooth behavior during accelerations and decelerations with fast torque changes. As such, increasing damping around the driveline resonant frequency is a typical task for all types of automotive powertrain controls. Automatic transmissions with hydraulic torque converters have a large natural viscous damping effect due to the torque transfer loss on the fluid. In HEV applications that do not include a torque converter or similar device, this natural damping effect is diminished. The resonant mode can be excited by the motor torque input due to the fast response of the electric machines and the small damping ratio in the mechanical driveline. The transient smoothness is largely dependent upon a well-designed control system.